This invention relates to a projector for projecting a full-color image on a screen and, more particularly, to a liquid crystal projector which uses three light valves as an image forming device to display images of three primary color lights, such as red, green, and blue lights, respectively, and which combines the red, the green, and the blue lights from those light valves into one full-color image light, and which projects this light.
The conventional projector comprises a light source, a cold mirror, a cut filter, a color separation optical section, a color combination optical section, three image forming devices, and a projection lens.
The color separation optical section separates a visible ray into a separated red light beam, a separated green light beam, and a separated blue light beam B. More specifically, the color separation optical section comprises a first color separating dichroic mirror for red transmission, a second color separating dichroic mirror for blue transmission. The first color separating dichroic mirror transmits the red light beam and reflects both the green light beam and the blue light beam. Both the green light beam and the blue light beam from the first color separating dichroic mirror enter the second color separating dichroic mirror. The second color separating dichroic mirror transmits the separated blue light beam and reflects the separated green light beam.
Each of three image forming devices comprises a condenser lens, a narrow band pass filter, and a light valve. The condenser lens converges the separated color light beam (the separated red, green, and blue light beams) which is perfect parallel light, and make amount of light to be entered to the projector lens increase so as to increase a brightness around a projector screen. The condenser lens transmits a converged color light beam (a converged red, green, and blue light beams). The narrow band pass filter narrows the wavelength band of the converged color light beam to transmit a narrowed color light beam (a narrowed red, green, and blue light beams) with a central wavelength. The narrowed color light beam enters the light valve. Thereafter, the light valve modulates the narrowed color light beam in accordance with a control signal from a driving circuit to form a red optical image. The light valve produces a color image light beam (a red, green, and blue image light beams) indicative of the optical image.
Each of the red, green, and blue image light beams enters the color combination optical section. The color combination optical section comprises a first color combination dichroic mirror and a second color combination dichroic mirror. The first combination dichroic mirror transmits the red image light beam and reflects the green image light beam. That is, the first color combination dichroic mirror combines the red image light beam with the green image light beam to produce a color combined image light beam to the second color combination dichroic mirror. The second color combination dichroic mirror transmits the blue image light beam and reflects the color combined image light beam. That is, the second color combination dichroic mirror combines the blue image light beam with the color combined image light beam to produce a final color combined image light beam to the projector lens.
As well known in the art, the spectral characteristic of the dichroic mirror varies in accordance with the incident angle of incident light.
That is, the first color combination dichroic mirror shows predetermined spectral characteristic to light entering on its center portion, while showing spectral characteristics shifted on the shorter wavelength side or longer wavelength side to light entering on either end portion. Therefore, the full-color image to be projected on the projection surface will inevitably have low brightness and irregular color.
To solve above problems, in the conventional projector, the narrow band pass filter for reducing the color nonuniformity of the projected image was provided. However, the conventional projector is complex in constitution and is high in cost.
Another conventional projector has been proposed by Tanaka and the others (U.S. Pat. No. 5,164,821) for reducing color nonuniformity of the projected image. The other conventional projector is similar to above-mentioned conventional projector except that three halfwave plates are disposed in the light-leaving sides or light-incident sides of three light valves.
A halfwave plate converts the direction of polarization so that the color light with p-polarized light component may pass through a light valve disposed on incident side, when incident light with (p+s)/2-polarized light component enters to the light valve. The (p+s)/2-polarized light component is called a mean component. The light valve uses the color light of the p-polarized light component and form an optical image. Furthermore, by rotating the halfwave plate, the direction of polarization may be continuously controlled, so that the spectral distribution characteristic of the color light entering the light valve may be controlled in a range from the p-polarized light component to the s-polarized light component.
A gradient width is defined as a difference between the wavelength when the transmittance is 10% and the wavelength when it is 90%. The gradient width is greater in the mean component than the p-polarized light component. The smaller the gradient width of the dichroic mirror, the smaller becomes the change of brightness and chromaticity of the color light on the light valve with respect to the change of the spectral transmittance of the dichroic mirror due to incident angle dependence.
Since the light from the light source is not a perfect parallel light, the incident angle of the light entering the dichroic mirror varies depending on the incident location of the incident light entering the dichroic mirror. The greater the incident angle, the shorter wavelength side the spectral characteristic is shifted to, and when the incident angle is smaller, it is shifted to the longer wavelength side.
This change of the spectral transmittance due to incident angle into the dichroic mirror is smaller in the color light of either one of the p-polarized light component or the s-polarized light component than the color light of the mean component. Therefore, the color nonuniformity of the projected image can be reduced by using either the p-polarized component and s-polarized component. However, in the other conventional projector, because the spectral characteristic of the dichroic mirror shifts in accordance with incident angle dependence continuously, the color nonuniformity of the projected image due to incident angle dependence occurs.